1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing a hot rolled steel strip. More particularly, the present invention relates to a method for manufacturing a hot rolled steel strip, the method capable of effectively removing iron oxide films from surfaces of a hot high temperature coil and having an improved total process rate, and an apparatus for carrying out the method.
2. Description of the Prior Art
Generally, as shown in FIG. 1, hot coils are manufactured by rolling a slab to a certain thickness through a continuous hot rolling mill, cooling the rolled steel strip on a run-out table to an appropriate temperature and then winding the cooled steel strip in coil form by a downcoiler. Immediately after winding, the hot coils have a temperature of about 500 to 600xc2x0 C. To cool these high temperature hot coils to an ambient temperature, the hot coils are piled up in a coil yard for 3 to 5 days to cool naturally. If necessary, the naturally cooled hot coils are subjected to a shape correction through a shape correction process, for example, a skin pass rolling mill.
The hot rolled steel product is supplied in various forms according to final uses. For example, the hot coil is directly usable as it is. Also, the PO (pickled and oiled) product, which is manufactured by removing oxide films formed on surfaces of the hot rolled steel sheet through a pickling process and then applying anti corrosive oil on the surfaces, may be supplied. The hot rolled steel sheet may be subjected to plating or cold rolling, after pickling. The surface-treated steel sheet product or cold rolled steel sheet product may be supplied.
Meanwhile, the high temperature hot coils drawn from the downcoiler must be cooled to 100xc2x0 C. or below before these hot coils are introduced into the shape correction process or pickling process. More particularly, since it is known that only when the shape correction process for a low carbon steel is performed at a temperature of 100xc2x0 C. or less, a coil breakage phenomenon can be prevented; thus, the hot coil must be cooled to the above temperature range.
However, it takes at least 3 to 5 days to naturally cool the hot coils of 500 to 600xc2x0 C. to 100xc2x0 C. or below, resulting in lengthening of the production period. Moreover, while the hot coils are slowly cooled over a long period, oxide films formed on surfaces of the steel sheets react with oxygen in air. Accordingly, the thickness of the oxide films is increased, and also compact and adherent oxide films such as Fe2O3 or Fe3O4 are formed, thereby making it difficult to perform the pickling process.
As a method for reducing the cooling time for hot coil, a forced cooling method, which is performed by injecting water onto wound hot coil or dipping the coil into water, is known, as disclosed in Japanese Patent Laid-Open Publication No Sho.63-20417, Sho.57-134207 and Sho.55-10355. However, in this conventional method, since water is merely in contact with outer surfaces of the hot coil, an external portion of the coil in contact with water is rapidly cooled, and while an internal portion of the coil not in contact with water has a cooling time of 6 to 24 hours. That is, this method cannot reduce a cooling time significantly. Also, this method has disadvantages in that it causes a deviation of mechanical properties caused by a difference in the cooling hysteresis between the internal and external portions of the coil and between both end portions and a center portion of the coil, and has a low cooling efficiency.
Korean Patent Disclosure No 1999-026910 discloses a method for solving the above problems. This method is a cooling technique that is carried out by winding steel sheets while interposing a steel strip between the steel sheets and then dipping the hot coil into water so that water is infiltrated between the steel sheets spaced apart at a constant interval by the steel strip. This method has an effect of largely decreasing the cooling time. However, the method has disadvantages of the inconvenience of winding the steel sheet together with the steel strip, and deterioration of shape quality caused by the steel strip.
Also, hot rolled steel strips covered with oxide films are first subjected to a oxide film removing process to manufacture PO steel, or to secondary treatment such as cold rolling or plating. Recently, a common method for removing the oxide films is a chemical pickling process as shown in FIG. 2. Chemical pickling is carried out as follows: a wound hot coil is continuously dipped into a strong acid aqueous solution such as hydrochloric acid or sulfuric acid, or injected with an acid aqueous solution while being unwound by an uncoiler. At this time, oxide films formed on surfaces of the hot coil are dissolved into the solution and then removed. Lastly, the hot coil is washed and dried.
However, since the chemical pickling reaction is comparatively slow, the process requires much time and thus has a low productivity. Furthermore, the process needs a large-scale facility including a very long pickling bath with a length of several tens of meters to achieve an appropriate feeding speed. Also, air pollution caused by acid vapors vaporized from the process, the deterioration of working environment, problems caused by corrosion of surrounding facilities and pollution problems caused by continuous generation of acid wastes cannot be avoided.
To improve the inefficiency of oxide film removal and environmental contamination caused by the chemical pickling as described above, various techniques have been suggested. Examples of those techniques include a method of stripping oxide films by a hydrolysis method using a neutral solution and a method for improving pickling by depressing the steel sheet using a roller disposed before a pickling bath, a surface modification using a scale breaker in the form of a leveler and crushing the oxide films through shot blasting. However, these methods are not a complete solution to the above problems and they complicate a facility. Also, a method for stripping oxide films by irradiation of a high-energy laser beam is suggested, but it has a difficulty in productivity and facility operation.
Also, Japanese Patent Laid-open Publication No Sho.57-1515 discloses another method for removing oxide films from a coil. In this method, after hot rolling, a high temperature steel strip coil is dipped into water as it is, and then cooled. Thus, on the surface of the steel sheet is formed a scale layer. Then, the coil is unwound in strips and the unwound steel strip is depressed to crush the scale layer. Thereafter, the crushed scale layer is removed by use of water jet, or strongly spraying an abrasive accelerated by air or water onto the surface of the steel strip.
Korean Patent Disclosure No 1998-048550 discloses a method for manufacturing a steel sheet, which is characterized in that a quenching zone and a dry type descaler are arranged behind a downcoiler of the hot rolling process, and the hot rolling process is connected online to the cold rolling process. This method reduces the cooling time of hot coils and solves problems of environmental contamination by using the pickling-free dry type descaling process. Also, this method can make the entire process continuous and simple.
However, since the process is continuous, it becomes difficult to carry out in the case where the characteristics of the process require a buffer to control flow of materials from the hot rolling process to the cold rolling process. Also, since the pickling-free dry type descaling process is not described in detail, the process is difficult to be realized.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a method for manufacturing a hot rolled steel strip, which can more efficiently remove oxide films formed on surfaces of the steel strip as well as sharply reducing cooling time.
It is another object of the present invention to provide a method for manufacturing a hot rolled steel strip, which has a reduced process time.
It is yet another object of the present invention to provide an apparatus for manufacturing a hot rolled steel strip, which can more efficiently remove oxide films formed on surfaces of the steel strip as well as improving cooling time and reducing process time.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a method for manufacturing a hot rolled steel strip free of oxide films comprising the steps of:
maintaining a steel strip coil at a high temperature as it is at a temperature of 400xc2x0 C. or more until the phase transformation is completed, after hot rolling;
water-cooling the steel strip coil at a speed of at least 50xc2x0 C./sec to 100xc2x0 C. or less while uncoiling the coil;
correcting the shape of the steel strip using a correction rolling mill;
removing oxide films formed on surfaces of the shape-corrected steel strip by injecting water jets to the surface; and
drying the steel strip free of oxide films and winding the steel strip.
In accordance with another aspect of the present invention, there is provided an apparatus for manufacturing a hot rolled steel strip comprising:
an uncoiler adapted to continuously supply a hot rolled steel strip coil with a high temperature while unwinding the coil in sheet form;
a quencher including a plurality of cooling-water headers arranged at an upper side and lower side of the steel sheet continuously supplied from the uncoiler, ventilating means for discharging a large amount of water vapor generated during cooling outward, and a plurality of table rollers for feeding the steel sheet, each header provided with nozzles so that the cooling water is discharged to one side of the steel sheet and connected to a cooling water supply source;
a correction rolling mill positioned downstream from the quencher and provided with a rolling roll set for imparting a desired thickness reduction to the steel sheet to eliminate a degradation in shape and non-uniformity of residual stress of the steel sheet fed from the quencher, and generate cracks on oxide film layers;
an oxide film remover positioned downstream from the correction rolling mill, the oxide film remover including pinch roll sets respectively disposed at an inlet and outlet of the remover and adapted to transmit a driving force for feeding the steel sheet, a plurality of guide roller sets arranged between the pinch roll sets while being in contact with upper and lower surfaces of the steel sheet and adapted to prevent the steel sheet from being sagged by its weight and hold the steel sheet so that the steel sheet proceeds at a desired level, means for injecting water jets disposed between the inlet and outlet pinch roll sets and adapted to inject water jets onto the steel sheet at upper and lower surface sides of the steel sheet, thereby removing the oxide films formed on the surface of the steel sheet, and a chamber enclosing the pinch roll sets, the guide roller sets and the water jet injecting means and having slits formed at inlet and outlet sides of the chamber and adapted to allow the steel sheet to pass therethrough;
the water jet injecting means including a pump for generating the water jet, cylindrical water jet headers connected to the pump and adapted to receive the water jets, the headers being arranged in the lateral direction of the steel sheet, and nozzles arranged in a line on each header and adapted to inject the water jets to the steel sheet at a desired inclination angle in the width direction;
a drier for drying the steel strip emerging from the outlet side slit of the oxide film remover; and
a recoiler for winding the steel sheet fed from the drier.
The chemical pickling method for removing oxide films formed on surfaces of a hot rolled steel strip has various problems in that it causes contamination, as well as requiring much time and complicated facilities. So, the present inventor studied to develop an improved method for mechanically removing oxide films, which can substitute the chemical pickling method. As the result, it was found that if a high temperature hot-rolled strip drawn from downcoiler would be cooled in an optimized manner, oxide films formed on surfaces of a hot rolled steel strip of the coil had a more coarse structure and was fragile, so that the oxide films could be easily removed by a subsequent mechanical method. Further, the total process speed for manufacturing the hot rolled steel strip can be largely improved due to the increased cooling rate. Based on these findings, the present invention has been completed.